In many known photoelectric synchronous detection systems, a pulsed optical beam signal is transmitted into a volume or zone of space being monitored, typically by using an LED which is activated by a square wave or low duty factor pulse generator/oscillator. An optical photodetector is aimed into the monitored zone with a field of view which includes the pulsed LED beam so that it will receive any reflection of that signal from an object in the zone to detect the presence of the object. Such a system is shown in U.S. Pat. No. 5,463,384--Juds.
To screen out noise and signals from sources other than a reflection of beamed light from the object in the zone, (e.g. from other electrical or optical sources), synchronous receivers are used to operate the receiver only when a reflection of the pulsed signal from an object in the zone is expected. This blocks any response resulting from detection of light energy from other sources during intervals when no reflected pulsed signal is possible.
To reject possible detection of intrinsic random circuit noise and detector shot noise, a fixed detection threshold is imposed on the system at a level above the expected intrinsic random noise levels seen by the detection circuit. This allows the detection circuit to ignore this noise. The probability of false detection due to noise is a function of the threshold level relative to the actual noise level, the amplitude of which is generally a Gaussian distribution.
Other examples of fixed threshold photoelectric detection systems are found in U.S. Pat. Nos. 4,356,393--Fayfield, 4,851,660--Juds, 4,851,661--Everett, Jr., 4,990,895--Juds, and 5,122,796--Beggs et al. Although these fixed threshold synchronous detection systems have been found useful for most photoelectric sensor applications, they are not sufficiently accurate in a situation where high receiver sensitivity is desired in an operating environment having a noise level that is highly inconsistent and randomly variable.
In such an environment, detector system performance is handicapped by the necessity of tailoring detection threshold levels to performing in an environment of the worst expected noise conditions to assure a satisfactory level of noise rejection. This situation exists when the detection system is used for vehicle detection in an outdoors operating environment. Such a system which is used to detect vehicles in a driver's blind spot will encounter a wide variation in noise resulting from ambient light conditions that range from pitch dark nighttime, to 8500 ft-cdls of sunlight reflected from a white surface, to as high as 70,000 ft-cdls of sunlight reflecting from a wet road surface. Since false detects by such systems renders them unreliable to a vehicle driver, elimination of false detects is an important goal.
In a blind spot detection system, the reflectivity of detected target vehicles will vary wildly, as will ambient lighting conditions. Thus, to be effective, such a system will be required to detect vehicles that range in reflectivity from black to white, and in lighting conditions that vary from pitch-dark nighttime to bright sunlight. These detection requirements range in the extreme from a black vehicle at nighttime to a white vehicle in bright sunlight.
In the dark of night very little DC photocurrent is produced in the detectors, resulting in very little shot noise. However, operation in bright daylight will result in quite significant DC current in the receiver photodiodes, resulting in high shot noise levels. The shot noise current produced by DC photocurrent in a silicon photodiode is determined by the equation: EQU i=5.66.times.10.sup.-10 .sup.- Idc.multidot.BW Amps RMS
where:
Idc is the DC photocurrent, and PA1 BW is the circuit bandwidth in Hz.
When the receiver views a white target vehicle in bright sunlight, the photocurrent generates shot noise which is many times greater than the intrinsic electronic noise of the receiver amplifier itself. To avoid false detection caused by a high level of shot noise, the required threshold must be quite large in comparison the worst case shot noise. This high threshold results in low system capability of detecting very dark, low reflective targets in poor lighting conditions.
There have been several attempts to overcome the operational problems caused by this wide variation in system noise levels. These involve providing the detection system with some form of adaptive adjustment based on a measurement of the noise amplitude characteristics which are then used to set the detection threshold of the receiver. The resulting adaptive threshold receiver optimizes its sensitivity relative to the ambient measured receiver noise to maintain signal reception integrity. Examples of such systems are found in U.S. Pat. Nos. 3,999,083--Bumgardner, 4,142,116--Hardy et al, 4,992,675--Conner et al, and 5,337,251--Pastor.
Such systems are quite expensive, since they require the addition of circuitry to continually measure noise, to block such measurement and maintain the prior measurement when an actual signal is detected, and to feed measured levels back to the variable gain stage. This circuitry adds components and assembly labor, thus increasing system size and cost.
There is a need for a detection system which automatically adapts the sensing threshold to changing ambient conditions to achieve optimum sensitivity, low probability of false detections under all ambient noise conditions, and requires no added components.
Vehicle blind spot detector systems such as disclosed in the above-mentioned patents utilize both driver-side and passenger-side detectors. One system comprises sets of six emitter-detector pairs in a module, the detectors being pairs of photodiodes of opposite polarity. The effective range of the system is determined by the geometry of these components. These components are quite small and require very precise manufacturing to maintain their geometry.
These modules also incorporate an emitter-detector pair comprising a so-called "dirty window" detector for determining when a transparent lens cover for the unit is too fouled with contaminants to enable effective vehicle blind spot detection. This emitter-detector pair monitors a portion of the cover that is spaced from the portions of the cover through which the emitter beams and detected reflections travel. The dirty windows detection threshold is dependent on the geometry of the components which are, again, dependent on very precise manufacturing to hold close tolerances.
There is a need for a blind spot detector that incorporates built-in adjustments for selectively varying range of operation and for assuring accurate dirty window detection.